Anatomical and physiological properties of the projection from the sensory cortex to the motor cortex in normal cats: the difference between corticocortical and thalamocortical projections.

Details of the distribution of terminal sites of the projection fibers from area 2 of the sensory cortex to the motor cortex were studied and compared with the distribution of terminals from the ventrolateral (VL) nucleus of the thalamus to the motor cortex. The results obtained were as follows: Intracortical microstimulation (ICMS) in area 2 produced measurable short-latency EPSPs only in neurons located in layers II and III of the motor cortex, whereas VL stimulation produced short-latency EPSPs in neurons throughout the depths of the motor cortex. The time from the beginning to the peak of the EPSPs was not significantly different for area 2- and VL-elicited EPSPs suggesting that there was no systematic difference between effective terminal sites for both inputs. However, there was a difference when a given neuron received both inputs suggesting that there was a segregation between the two inputs within a given cell. The majority of area 2-elicited EPSPs were smooth and monophasic, but some (40%) of them showed double peaks indicating that some neurons received mono- and disynaptic inputs from area 2. Intracellular injections of HRP suggested that neurons receiving input from area 2 were predominantly multipolar non-pyramidal neurons in layers II and III whereas neurons receiving thalamic input were pyramidal as well as non-pyramidal cells. Field potentials in the motor cortex evoked by area 2 stimulation did not change polarity in the depths of the cortex and therefore, differed from the VL-evoked potentials suggesting differences in the mechanisms of generating the electrical fields. It is concluded that association fibers effective for producing EPSPs terminate primarily on non-pyramidal cells in layer II and III whereas VL fibers terminate not only on pyramidal but also on non-pyramidal cells in layers III and V. This study provided a basis for examining the modifiability of association fibers after elimination of VL input to the motor cortex which is reported in the following paper.